Ceramics comprising silicon nitride as the main component are superior in heat resistance, mechanical strength, and toughness to other ceramic materials, and are materials suitable for various structural parts such as automotive parts and OA apparatus parts. Attempts are being made to use them as insulating radiating substrates for semiconductor devices, etc. so as to take advantage of their high insulating properties.
Alumina and the like have conventionally been used extensively as ceramic substrates for semiconductors. However, with the trend toward higher speeds, higher degrees of integration, and higher outputs in semiconductor devices, materials having higher thermal conductivity and excellent radiating properties have come to be desired and the application of AlN and SiC has progressed. However, no high thermal conductive substrate has been obtained so far which is made of AlN or the like and is sufficient in strength and toughness, and the current substrates have drawbacks in product handling and shape because of breakage caused by external force, etc. There is hence a desire for the development of a ceramic material combining high-strength properties which enable the material to withstand external force with excellent radiating properties.
Silicon nitride (Si.sub.3 N.sub.4), which intrinsically has high strength, is expected to be used as insulating radiating substrates if its thermal conductivity can be improved. However, since the conventionally known silicon nitride sintered bodies have lower thermal conductivities than AlN and SiC, they have not been put to practical use as an insulating radiating substrate.
The thermal conductivity of insulating ceramics such as silicon nitride is mainly attributable to the transmission of phonons. Since phonons are scattered by phases having different impedances, such as lattice defects and impurities, present in the sintered body, the thermal conductivity .kappa. is defined by the following numerical formula 1: EQU .kappa.=c.times.V.times.l/3 (Numerical formula 1)
(wherein c is specific heat capacity; V is average velocity of phonons; and l is the mean free path of phonons).
The specific heat capacity c and the group velocity V in numerical formula 1 each is a number which varies from material to material and can be regarded as almost the same in the same material. Consequently, the thermal conductivity of silicon nitride crystal grains is governed substantially by the mean free path of phonons. For example, when AlN or Al.sub.2 O.sub.3, which have conventionally been used generally, is added as a sintering aid, then aluminum ions or oxygen ions form a solid solution in Si.sub.3 N.sub.4 crystal grains and thus scatter phonons, resulting in a reduced thermal conductivity. Because of this, general silicon nitride base sintered bodies to which Al.sub.2 O.sub.3, AlN, Y.sub.2 O.sub.3, or the like has been added have a thermal conductivity as low as about 15 W/m.multidot.k.
Various investigations have hence been made in order to obtain a silicon nitride base sintered body having a high thermal conductivity. For example, the thermal conductivity of a silicon nitride base sintered body obtained through an HIP treatment after the addition of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 in combination as a sintering aid is discussed in "Paper Journal of Ceramics Society of Japan)," Vol.97 (1989), No.1, pp.56-62. The result given therein is that the thermal conductivity of the sintered body becomes higher as the proportion of .beta.-form crystal grains increases or as the proportions of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 in the sintering aid increases and decreases, respectively. There is a description in the paper, section 4.2 to the effect that high thermal conductivity is obtained by .beta.-form crystal grains because .beta.-form crystal grains have a larger mean free path of phonons than .alpha.-form crystal grains.
It is therefore important for heightening the thermal conductivity of a silicon nitride base ceramic to accelerate the formation of .beta.-form Si.sub.3 N.sub.4 crystal grains, to use a rare earth element compound such as Y.sub.2 O.sub.3, which is regarded as less apt to form a solid solution in the crystal grains, and to diminish the addition of an aluminum compound containing aluminum ions, which are apt to form a solid solution in the crystal grains.
For example, Japanese Patent Laid-Open Nos. 175268/1992 and 219371/1992 show a case in which a dense silicon nitride base sintered body having a thermal conductivity of 40 W/m.multidot.k or higher and consisting of .beta.-form Si.sub.3 N.sub.4 crystal grains was obtained by using a .beta.-form Si.sub.3 N.sub.4 powder reduced in the contents of oxygen and cationic impurities so as to diminish the amounts of cationic impurities such as aluminum and oxygen, which form a solid solution in Si.sub.3 N.sub.4 crystal grains, and by additionally adding a compound of, e.g., a Group 4A element when a colored sintered body was to be obtained.
Japanese Patent Laid-Open No. 30866/1997 discloses a dense silicon nitride base sintered body having a thermal conductivity of 80 W/m.multidot.k or higher and a flexural strength of 600 MPa or higher which is obtained by adding a compound of an alkaline earth/rare earth element and conducting sintering in high-pressure nitrogen gas at a relatively high temperature around 2,000.degree. C. to thereby heighten the proportion of large .beta.-form crystal grains having a minor diameter of 5 .mu.m or larger.
Japanese Patent Laid-Open Nos. 135771/1994 and 48174/1995 disclose a method for obtaining a dense silicon nitride base sintered body consisting of .beta.-form crystal grains which comprises adding an appropriate amount of aluminum ions together with a rare earth element compound and a Group 4A element compound, without limiting the amount of aluminum ions, and gradually cooling the shape after sintering to thereby accelerate crystallization in the grain boundary phase. There is a description in these patent documents to the effect that an Si.sub.3 N.sub.4 base sintered body having a flexural strength of 800 MPa or higher and a thermal conductivity of 60 W/m.multidot.k or higher is obtained.
On the other hand, Japanese Patent Laid-Open Nos. 149588/1995, 319187/1996, and 64235/1997 disclose: a metallized substrate comprising a silicon nitride base and formed thereon a high-melting metallizing layer made of tungsten or molybdenum; and a semiconductor module comprising the substrate and a conductor circuit bonded thereto. Japanese Patent Laid-Open No. 187793/1995 discloses various semiconductor devices containing a similar metallized substrate and various structural members comprising the same silicon nitride base sintered body. The above high-melting metallizing layer is one formed on the base through an oxide film made of SiO.sub.2, a layer of one or more Group 4A metals or of a brazing material containing these, or through a Cu--Cu.sub.2 O eutectic layer, and has a peel strength of 3 kgf/mm.sup.2 or higher.
As described above, it is important in the conventional methods to use a high-purity Si.sub.3 N.sub.4 powder reduced in the contents of oxygen and cationic impurities and to add an appropriate kind of sintering aid in an appropriate amount in order to inhibit oxygen ions and cationic impurities from forming a solid solution in crystal grains. Namely, in order to obtain crystal grains in which impurities or defects have been diminished, the purity of the grains should be increased by using expensive high-purity powder feedstocks as the main and minor ingredients and causing grain growth at a high temperature and a high pressure. For example, as described in Japanese Patent Laid-Open No. 30866/1997, cited above, it is necessary to employ a method in which high-purity .beta.-form silicon nitride is used as a feedstock powder and grain growth is caused at a high temperature and a high pressure (2,000.degree. C., 300 atm).